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As the Arctic warms, nor'easters may get worse

Study finds major winter storms were two to four times more likely when Arctic is abnormally warm

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Updated: 12:03 PM EDT Mar 21, 2018

As the Arctic warms, nor'easters may get worse

Study finds major winter storms were two to four times more likely when Arctic is abnormally warm

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Updated: 12:03 PM EDT Mar 21, 2018

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As the Arctic warms, nor'easters may get worse

Study finds major winter storms were two to four times more likely when Arctic is abnormally warm

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Updated: 12:03 PM EDT Mar 21, 2018

As the Northeast endures its fourth nor'easter in three weeks, winter-weary residents are wondering what's behind the weather chaos.

Evidence is mounting that the answer could lie in the rapidly melting Arctic.

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A study released last week in the scientific journal Nature Communications ties extreme winter weather, specifically major snowstorms in the northeastern U.S., to warming Arctic temperatures.

The study, from Atmospheric and Environmental Research (AER) and Rutgers University, zeroed in on U.S. cities and looked at how severe winter weather, extreme cold and heavy snowfall related to conditions in the Arctic during and before the storms.

The results found that major winter storms were two to four times more likely when the Arctic is abnormally warm, compared to when it was abnormally cold.

This connection was especially strong for locations in the eastern third of the U.S., while the Rockies and locations on the West Coast showed a weaker or even opposite relationship. This includes many of the most populated cities in the country, including New York City and Washington.

Building on a theory

It seems counterintuitive that a warming Arctic and melting sea ice would lead to colder, snowier winters in the middle latitudes of the Northern Hemisphere -- but the "warm arctic, cold continents" theory has been a hot topic in climate research for the past several years.

While other studies have looked at seasonal time scales and shown some correlation over the past couple of decades, this study looked at daily data to attempt to find a stronger connection.

"Because we could perform analysis on over 6,000 data points in comparison to less than 30 data points in previous studies we could show a much more robust (and statistically significant) relationship between a warm Arctic and increased severe winter weather in the mid-latitudes," Judah Cohen, director of seasonal forecasting at AER and lead author of the study told CNN.

And while other studies showed a relationship in temperatures, Cohen believes this study is first to show increases in heavier snowfall as well.

"As far as I know, for the first time we show that a warmer Arctic is also more favorable for heavy snowfall," Cohen said.

Examining the data also revealed that major cold air outbreaks and heavy snowfalls occurred more frequently in mid to late winter, when the abnormally warm air was found to extend all the way up to the upper atmosphere and likely disrupt the jet stream.

Recent winters have seemed to follow the same script.

"Five of the past six winters have brought persistent cold to the eastern U.S. and warm, dry conditions to the west, while the Arctic has been off-the-charts warm," added Cohen's coauthor Jennifer Francis, a professor at Rutgers University.

"Our study suggests that this is no coincidence. Exactly how much the Arctic contributed to the severity or persistence of the pattern is still hard to pin down, but it's becoming very difficult to believe they are unrelated."

This has certainly been the case this winter, with the "Beast from the East" that brought massive snows and frigid temperatures to Europe and the incredible string of strong nor'easters hammering the U.S. East Coast -- although this study only covered the years 1950 to 2016.

"The publication of the paper is especially timely given the extreme winter of 2017/2018: record warm Arctic and low sea ice, record-breaking polar vortex disruption, record-breaking cold in both the U.S. and Europe, disruptive snowfalls in both the U.S. and Europe, severe "bomb cyclones," costly nor'easters," Cohen said.

Cohen also provided the following graph, which shows "that winter of 2017/2018 is a textbook case of what we wrote about in the paper."

Judah Cohen via CNN

Time-series from the winter of 2017-2018 showing how warm periods in the Arctic relate to extreme winter weather events at the mid-latitudes.

Climate change and the polar vortex

It is important to note that this was a data-analysis study, meant to derive a correlation between Arctic warmth and extremes in winter weather in the United States -- but correlation does not mean causation. However, the connection appears to be too strong to ignore.

"Our statistical analysis shows that one is more likely to be struck by lightning, attacked by a shark and win the Powerball all at the same time than the possibility of severe winter weather in the northeastern US not being related to Arctic temperatures," Cohen said in a release.

While the study did not explain how the warmer Arctic is causing the more frequent and more extreme winter storms such as the recent strong of nor'easters, it did offer some possibilities that are supported by their data.

Most simply, reduced sea ice and warmer arctic temperatures lead to a weakening of the polar vortex -- the real one in the stratosphere, not the catchy name that has been applied to cold air outbreaks in recent years.

Think of the polar vortex as a belt of very strong winds in the upper level of the atmosphere. When the vortex is strong, it keeps the bitter arctic cold bottled up in the polar regions. But when it weakens, the cold air can spill out and the jet stream can dip much further south, bringing with it the cold air and heavy snow.

This is called the Arctic oscillation, something scientists have known about for decades. When the polar vortex is 'wavy,' that is the negative phase of the oscillation.

But recent trends cannot be explained by the Arctic oscillation alone.

"During this time period in December 2017 and early January 2018, the AO was positive," Cohen told CNN.

In fact, the correlation with severe winter weather was stronger with the Arctic warmth parameters used in the study than with the traditional Arctic oscillation.

But climate models do not handle this relationship very well, which makes explaining how a warming climate is resulting in colder and snowier winters more difficult. In fact, the most reliable climate models indicate that cold extremes and heavy snowfall should decrease as the Arctic continues to warm.

Determining the exact mechanism that is causing the warmer Arctic to bring about these disruptive and costly winter storms will be the focus of Cohen and his team going forward, as well as many in the field of climate change research.

This is especially the case in light of the record-breaking extreme weather seen in the first few months of 2018 across Europe and the United States.

"This study highlights the difficulty in disentangling the cause-and-effect between Arctic warming and middle latitude extreme events," Ryan Maue, a meteorologist with weather.us who was unaffiliated with this research told CNN.

"While no firm scientific consensus exists in the climate community on these Arctic interactions, this research communication will help direct future research and spur timely debate on a high impact climate change problem,' Maue said.